In recent years, instead of using aluminum or aluminum alloys as a material for forming interconnection circuits on a substrate such as a semiconductor wafer, there is an eminent movement towards using copper (Cu) which has a low electric resistivity and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in the surface of a substrate. Various techniques are known for forming such copper interconnects, including CVD, sputtering, and plating. According to any such technique, a copper film is formed in the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 1A through 1C illustrate, in sequence of process steps, an example of forming such a substrate W having copper interconnects. As shown in FIG. 1A, an insulating film 2, such as a silicon oxide film of SiO2 or a film of low-k material, is deposited on a conductive layer 1a in which electronic devices are formed, which is formed on a semiconductor base 1. A contact hole 3 and a trench 4 for interconnects are formed in the insulating film 2 by the lithography/etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the entire surface, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5.
Then, as shown in FIG. 1B, copper plating is performed onto the surface of the substrate W to fill the contact hole 3 and the trench 4 with copper and, at the same time, deposit a copper film 6 on the insulating film 2. Thereafter, the copper film 6 and the barrier layer 5 on the insulating film 2 are removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact hole 3 and the trench 4 for interconnects and the surface of the insulating film 2 lie substantially on the same plane. An interconnection composed of the copper film 6 as shown in FIG. 1C is thus formed.
Components in various types of equipment have recently become finer and have required higher accuracy. As sub-micro manufacturing technology has commonly been used, the properties of materials are largely influenced by the processing method. Under these circumstances, in such a conventional machining method that a desired portion in a workpiece is physically destroyed and removed from the surface thereof by a tool, a large number of defects may be produced to deteriorate the properties of the workpiece. Therefore, it becomes important to perform processing without deteriorating the properties of the materials.
Some processing methods, such as chemical polishing, electrolytic processing, and electrolytic polishing, have been developed in order to solve this problem. In contrast with the conventional physical processing, these methods perform removal processing or the like through chemical dissolution reaction. Therefore, these methods do not suffer from defects, such as formation of an affected layer and dislocation, due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials.
A processing method provided with an ion exchanger has been developed as electrolytic processing. FIG. 2 illustrates the principle of this electrolytic processing. FIG. 2 shows the ionic state when an ion exchanger 12a mounted on a processing electrode 14 and an ion exchanger 12b mounted on a feeding electrode 16 are brought into contact with or close to a surface of a workpiece 10, while a voltage is applied via a power source 17 between the processing electrode 14 and the feeding electrode 16, and a liquid 18, e.g. ultrapure water, is supplied from a liquid supply section 19 between the processing electrode 14, the feeding electrode 16 and the workpiece 10. In the case of this electrolytic processing, water molecules 20 in the liquid 18 such as ultrapure water are dissociated efficiently by using the ion exchangers 12a, 12b into hydroxide ions 22 and hydrogen ions 24. The hydroxide ions 22 thus produced, for example, are carried, by the electric field between the workpiece 10 and the processing electrode 14 and by the flow of the liquid 18, to the surface of the workpiece 10 opposite to the processing electrode 14 whereby the density of the hydroxide ions 22 in the vicinity of the workpiece 10 is enhanced, and the hydroxide ions 22 are reacted with the atoms 10a of the workpiece 10. The reaction product 26 produced by this reaction is dissolved in the liquid 18, and removed from the workpiece 10 by the flow of the liquid 18 along the surface of the workpiece 10. Removal processing of the surface of the workpiece 10 is thus effected.
When carrying out electrolytic processing of e.g. copper by using e.g. a cation exchanger having cation-exchange groups, copper is captured by the cation-exchange groups. Progress of the consumption of cation-exchange groups by copper makes it impossible to continue the electrolytic processing. When electrolytic processing of copper is carried out by using as an ion exchanger an anion exchanger having anion-exchange groups, on the other hand, fine particles of a copper oxide are generated and the particles adhere to the surface of the ion exchanger (anion exchanger). Such particles on the ion exchanger can contaminate the surface of a next substrate to be processed.
It is therefore considered to regenerate such consumed or contaminated ion exchangers in order to remove the above drawbacks. Regeneration of an ion exchanger is made by exchange of an ion captured by the ion exchanger for hydrogen ion in the case of a cation exchanger or for hydroxide ion in the case of an anion exchanger.
Ion-exchange processes using an ion exchanger are widely utilized for various purposes, such as purification, separation and condensation. Regeneration of an ion exchanger has conventionally been practiced by immersing the ion exchanger in an acid solution when the exchanger is a cation exchanger, or in an alkali solution when the exchanger is an anion exchanger. In the case of a cation exchanger which has captured an ion having an ion selectivity coefficient close to that of hydrogen ion, such as sodium ion, the ion exchanger can be regenerated in a very short time by immersing it in an acid solution. However, when an ion exchanger, which has captured an ion having a large ion selectivity coefficient, is regenerated by immersing it in an acid or alkali solution, the regeneration rate is very slow. Further, such a chemical liquid remains at a high concentration in the regenerated ion exchanger, requiring cleaning of the ion exchanger. In addition, disposal of the chemical liquid used in the regeneration is needed.
An ion exchanger to be contacted with a workpiece is generally in the shape of a thin film from the viewpoint of surface smoothness and flexibility. Accordingly, the ion-exchange capacity, which is a measure of processing amount, is generally small. It has therefore been practiced to laminate an ion exchanger having a large ion-exchange capacity between a film-type ion exchanger and an electrode so that most of the processing products may be taken in the laminated portion (laminated ion exchanger). Even with such a laminated ion exchanger, when the processing progresses to a certain extent, the laminated portion cannot take in the processing products any more. Change or regeneration of the ion exchanger is therefore necessary. Change of the ion exchanger is generally practiced by hand, and therefore a considerable time is needed for the exchange operation. When carrying out regeneration of the ion exchanger, processing must be stopped during the regeneration operation carried out by the conventional method, which adversely affects the throughput of the apparatus.